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Update pithcat to initialize the Pithos ModularBackend differently,
based on the type of the requested image URL. Specifically, in case the
image URL if a 'pithosmap' URL, no access to the database is needed. So
in this case, the backend must be initialized with 'None' database
connection URL.

Modify pithcat to be able to parse and work with URLs of the form:
`pithosmap://hash/size' which are neeed for archipelagos.

This is done for security reasons. The options may contain sensitive
data like the Pithos DB password and while the program (pithcat) is
running the whole command line is exposed to everyone through ps.

snf-image will detect at runtime if pithos backend is installed

It connects to a Pithos backend and either fetches
the data of a file or returns its size.

Refs #1745